The invention is particularly related to the form of television tuning systems which selectively tune to desired television channels by deriving channel number information indicative of the system tuning and adjust the tuning as a function of the difference between the derived and desired channel number information. Such systems are relatively new. U.S. Pat. No. 3,961,266 discloses a television tuning system which employs a comparator for comparing the asynchronously sampled local oscillator frequency, encoded in terms of channel numbers and a modular residue corresponding to intra-channel fractions, with encoded input channel number information. A voltage ramp drives the tuner until a condition of equality exists between the derived and desired channel numbers and the modular residue (modulo 6) is within prescribed limits. This tuning system not only provides equal access all-channel tuning but also includes a channel recall or memory system.
The memory system is completely described in the above-mentioned U.S. Pat. No. 3,961,266 and is additionally disclosed and claimed in above mentioned U.S. Pat. No. 3,946,319. The memory may be programmed in any desired sequence to provide sequential tuning among a selected group of channels. The advantages of such a tuning system with a memory are readily apparent since the viewer can sequence through only desired channels. The memory has sufficient capacity to hold the digits of the desired channel number normally in binary encoded decimal form. Eight bits of information are used in the memory for each desired channel.
U.S. Pat. No. 3,931,579 discloses a television tuning system which also derives channel number information from the local oscillator frequency and compares this with desired channel number information. Rather than requiring a channel number input it includes channel counters operating sequentially to generate desired channel numbers. As each channel number is generated the system responds by tuning to that frequency location. Additional means are included to determine if a broadcast signal is present at the channel location. If no signal is present the next sequential channel number is generated and the process repeated. If, however, a broadcast signal is detected the generation of sequential channel numbers is interrupted.
The above use of channel counters to generate channel number information greatly simplifies the tuning system for the viewer. He need only touch a single button corresponding to advancing the counters to higher or lower channel numbers to provide tuning. It is not necessary that he enter a 2 digit channel number at a keyboard. Although generating channel numbers is very convenient no provision is made for storing tuning information. Such television systems lack a convenient method of selecting from all possible channels only those channels which the viewer desires. Although a memory of the type which could store individual binary encoded channel number digits could be added to such a system, this addition would cause a considerable increase in cost because of the magnitude of information which must be stored.
The present invention relates to a novel apparatus and method of predetermining a sequence of channel numbers for use in tuning systems of the derived-desired channel number comparison type which sequentially generate channel numbers. It includes a memory with a capacity which, at maximum, can accommodate information related to the preselection of the total number of allocated television channels. This new and novel tuning system combines the features of automatically sequencing through all channel numbers with the option of selecting a smaller number of television channels for convenient or more frequent viewing. Because the tuning system does not store actual tuning information for its operation, the memory need only contain signals corresponding to preselected channels. Thus, the system operation is simplified and a less expensive meory may be utilized.